Gas-tight and oxidation resistant silicon nitride materials are often required in machines and in plants in recuperators, pipe lines, tanks, etc., wherein there are high application temperatures. There is interest in improving the oxidation resistance of silicon nitride materials used where gas tightness is absolutely necessary, for example, in combustion chambers, turbine blades and distributors.
Processes are known for producing gas-tight silicon nitride and for reducing its porosity.
"Powder Metallurgy International", vol. 6 (1974) p. 17 to 19 describes hot pressing silicon nitride with additives. Relatively great gas tightness is achieved. However, this process is only of interest for a limited number of applications because of the high costs and the limitation of the size and shape of the molded bodies.
Another known process consists of infiltrating or impregnating reaction-sintered silicon nitride with metal solutions or acids. The impregnated materials are converted into metal oxides inside the pores by temperature treatments, as suggested in German Offenlegungsschriften Nos. 2,360,343 and 2,351,162, but the material obtained has much poorer resistance to sudden temperature changes, as compared with pure silicon nitride. Additionally, this process is very energy-consuming since several successive infiltration-separation cycles are required to obtain the desired gas tightness.
It is also known to seal the surface of porous silicon nitride by applying glazes or coatings thereto, as described in German Offenlegungsschriften Nos. 2,041,587; 2,152,006; 1,646,796 and British Pat. No. 1,151,475. Such protective coatings crack easily during subsequent temperature treatments because the coefficients of thermal expansion of the silicon nitride and of the coatings differ considerably. Besides, it is technically extremely difficult to uniformly glaze or coat silicon nitride parts having complicated shapes or forms, e.g. recuperator blocks with very narrow long channels. Glazes customarily used in this field have the additional disadvantage that they either become sticky at temperatures below 1000.degree. C, or they melt at such high temperatures that the silicon nitride material is dissolved with resultant evolution of nitrogen, which leads to a poorly adhering glaze.
German Offenlegungsschrift No. 2,132,623 describes a process for improving the strength of silicon nitride bodies by controlled oxidation.
U.S. Pat. No. 3,455,729 discloses improving the resistance of silicon nitride bodies to damage due to sudden temperature changes by treatment of the silicon nitride with lithium vapors.
It is an object of the invention to provide a silicon-nitride containing material with a tight surface and improved resistance to oxidation by means of additives. It is another object of the invention to provide a simple and economical process for producing the silicon-nitride containing material.